Heterodyne oscillator



June 6, 1950 I l. R; VIVORSLEY 2,510,165

HETERUDYNE OSCILLATOR Filed May 20, 1947 {DHPI l6 33 55/7 343 l 27 4o- L 30 I nuentor 1V0? Ram WELL h/MSLEY Atlor y Patented June 6, 1950 HETERODYNE OSCILLATOR Ivor Rothwell Worsley, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 20, 1947, Serial No. 749,324 In Great Britain May 4, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires May 4, 1965 4 Claims.

The present invention relates to improvements in electric oscillation generators, and it concerns in particular neutralising arrangements for preventing or reducing the pulling-in which may occur in some types of variable frequency heterodyne oscillators when an attempt is made to adjust the output beat frequency below a limiting low value.

The term pulling-in is applied to the effect due to coupling between the circuits of the two component oscillators which causes them both suddenly to oscillate at the same frequency when the frequency of one of them is adjusted to differ by less than a certain small amount from the frequency of the other. This effect sets a lower limit to the output beat frequency of the oscillation generator, and any attempt to reduce the frequency below this limit causes complete suppression of the output.

When space and cost are important considerations in the design of a heterodyne oscillator, it is convenient to employ a multi-electrode valve which combines in the same envelope a triode valve to serve for one of the component oscillators, and also the modulating valve in which the output waves of both oscillators are heterodyned in order to produce the desired difference frequency. When this is done it is found that the capacity between the two anodes of the multielectrode valve is a serious cause of coupling between the two oscillators which produces pullingin. This coupling arises from the fact that waves at the frequency generated by the separate oscillator valve will be present on the anode of the modulating valve and will be fed through the aforesaid capacity and through the oscillation circuit associated therewith; and thence to the control grid of the triode which is in the same envelope as the modulating valve.

According to the invention this source of pulling-in is removed by connecting a condenser of suitable capacity between the anode of the modulating valve and a point in the oscillation circuit of the associated triode in such manner as to reduce substantially to zero the voltage variations applied between the control grid and anode of the triode as a result of the voltage variations of the modulating anode.

The invention will be described with reference to the figure of the accompanying drawing which shows a schematic circuit diagram of an embodiment of the invention. This figure shows a multi-'- electrode valve I comprisin two valve sections sharing a common cathode 2 connected to ground. One of these sections is a triode having a control grid 3 and an anode 4, and the other section is a modulator having two control grids 5 and 6 separated by a screen grid I, and an anode 8. The triode 2, 3, 4 is associated with an oscillation circuit adapted to generate a frequency f1; and a separate triode valve 9 having a. cathode l0 connected to ground, a control grid II, and an anode I2, is associated with a second similar oscillation circuit adapted to generate a frequency f2.

The oscillations at these frequencies are applied respectively to the control grids 5 and 6 of the modulator section of the valve I and the modulation productsare taken from the anode 8; the difference frequency fi-fz is selected by a suitable filter (not shown) and amplified in any convenient manner (also not shown).

The oscillation circuit for the frequency f1 comprises an inductance I3 connected between the anode 4 and the control grid 3, the connection to the control grid being made through a blocking condenser I4. The usual resistance I5 connects the control grid 3 to ground. The inductance I3 is tapped at an intermediate point for the supply of anode current to the triode from the positive high tension terminal I6 through a resistance I'I, associated with a by-pass condenser I8. The negative high tension terminal I9 is connected to ground. A condenser 20, which may be adjustable, shunts the coil l3 and forms therewith a resonant circuit tuned to the frequency ii. A winding 2| inductively coupled to the coil l3 has one terminal connected to the control grid 6, and the other terminal is connected through an appropriate negative grid biassing source 22 to ground. The source 22 may be provided in any known way and is shunted by a by-pass condenser 23.

The oscillation circuit for the frequency 12 associated with the valve 9 has elements 24, 25, 26. 21, 28, 29, 30, 3I and 32 corresponding respectively with the elements I3, I4, I5, l1, I8, 20, 2|, 22 and 23, and associated in the same way with the corresponding oscillating valve electrodes, the winding 30 being connected to the control grid 4 of the valve I. The bias applied to each of the control grids 4 and 6 should be adjusted in order to obtain suitable modulating characteristics, and the numbers of turns of the windings 2| and 30 should be chosen so that appropriate voltages are applied to the two control grids. These details will be understood 'by those skilled in the art. The biassing arrangements shown are conventional and may be provided in any suitable way.

Thecondenser 29 may be a variable condenser, by which the output beat frequency is adjusted to any desired value.

The anode 8 is connected to the high tension terminal I6 through two resistances 33 and 34, the junction point of which is connected to ground through a by-pass condenser 35, forming a decoupling arrangement. The screen grid 7 is polarised through a resistance 36 connected to the terminal I6, a by-pass condenser 3'! being provided as usual. The anode 8 is connected to an output terminal 38 by which the modulation products are conveyed to conventional filtering and amplifying means (not shown).

The heterodyne oscillator which has been described so far is a well known arrangement except that one of the oscillating triodes is combined in the same envelope with the modulating valve. As already explained, the capacity of the condenser shown dotted at 39 between the two anodes d and 8 is not negligible and may be the cause of pulling-in. The oscillation voltage of the anode 8 contains components at both frequencies f1 and f2 as well as various other sum and difference frequencies, and a current including the frequency f2 will be fed through the con denser 39, lower part of the coil I3 and through the by-pass condenser I8 to ground. This will produce a difference of potential between the anode 4 and grid 3 so that when the condenser 29 is adjusted so that f2 is very nearly equal to f1, a point is reached at which the frequency f2 takes control and forces the triode 2, V3, 4 to oscillate at f2. To prevent this effect according to the present invention, a condenser 40 is connected between the anode 8 and the upper end of the coil I3, which end is connected :to the control grid 3 through the condenser lI- l. In this way a Wheatstone bridge is formed of which the arms comprise the two sections of the coil I3 and the two condensers 39 and 40.

If the capacity of the condenser 4|! is adjusted so that the bridge is balanced, the potential difference between the electrodes 3 and 4 due to the current flowing to ground from the anode 8 through the condenser I8 will be reduced to zero and so this source of pulling-in is removed. In the particular case where the .coil I3 is tapped at the centre, the capacity of the condenser 48 should be equal to the capacity between the two anodes 5 and 8.

The invention is not confined to the particular circuit arrangement shown in the accompanying figure. Different types of oscillation circuits may be used, and current supply and biassing arrangements and the like may be provided in any appropriate way, depending on the types of valves used. Suitable extrascreening arrangements not indicated in the figure may be provided.

What is claimed is:

1. An electric oscillation generator of the heterodyne type comprising two component oscillation circuits each including an oscillator valve, and means including a modulating valve for deriving from the two circuits waves having a frequency equal to the difference between the fre- .said modulating valve to a point in the oscillation circuit associated with the last mentioned oscillator valve, the capacity of the condenser and the said point being so chosen as to reduce substantially to ,zero the alternating difierence of potential between the control grid and anode of the last mentioned oscillator valve which results from the potential variations of the anode of the modulating valve.

2. An electric oscillation generator of the heterodyne type comprising a first oscillator valve enclosed in the same envelope with a modulating valve having two control grids, a cathode and an anode, a second oscillator valve, each of said oscillator valves comprising an anode and a control grid, an inductance shunted by a tuning condenser effectively connected between the anode and control grid of each oscillator valve, an individual output winding coupled to each of the said inductances and connected so as to supply oscillation potentials between the cathode and a corresponding one of the control grids of the modulating valve, means for supplying anode current to each of the oscillator valves through a tapping point on the corresponding inductance, means for selecting waves of the difference frequency from the anode of the modulating valve, and a neutralising condenser connecting the anode of the modulating valve efiectively to the control grid of the first oscillator valve, the capacity of the neutralising condenser being adjusted to such a value as to prevent pulling-in resulting from coupling through the capacity between the anode of the first oscillator valve and the anode of the modulating valve.

3. A generator according to claim 2 in which the tapping point is at the centre of the corresponding inductance, and in which the capacity of the said neutralising condenser is substantially REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,197,395 Schlesinger Feb. 8, 1938 2,122,283 Harris June 28, 1938 2,262,1a9 Slonczewski Nov. 11, 1941 2,422,742 Odessey June 24, 1947 

